466 



THE COMMUNITY 



lians and many burrowing frogs), reptiles 

 (certain limbless lizards and many 

 snakes), birds (the burrowing owls and 

 bank swallows), and a wide variety of 

 mammals. Such mammals are especially 

 well adjusted for life in this stratum 

 (Shimer, 1903; Lull, 1920; Hesse, Allee, 

 and Schmidt, 1937, p. 423). Others feed 

 upon the herbaceous stratum, travel on the 

 floor stratum, and excavate burrows into 

 the subterranean stratum for the brooding 

 of their young, or as a habitat niche for 

 daily physiological recuperation, or as a 

 protection against both adverse weather and 

 natural enemies. Others— for example, the 

 American badger— dig into the subterra- 

 nean stratum or pursue burrowing prey. 

 Finally, such animals as swine travel on the 

 floor stratum and dig for a part of their 

 food in the subterranean stratum. Although 

 many species are more or less fossorial, 

 only a few mammals are, strictly speaking, 

 inhabitants of the subterranean stratum; 

 most of the larger vertebrates utilize sev- 

 eral strata in the vertical gradient, as do 

 certain bottom-feeding fishes already men- 

 tioned, and hence may be thought of as 

 binding species in the community. 



STRATIFICATION IN GRASSLAND 

 COMMUNITIES 



Grassland in general is limited to terres- 

 trial areas with an annual precipitation of 

 between 20 and 40 inches, little of which 

 is deposited in the hot season, and by the 

 edaphic characteristics of the soil and the 

 population of grazing animals. Grassland, 

 limited by climatic, edaphic, and biotic 

 factors, includes diverse communities at the 

 level of self-sustenance. Despite differences 

 in the area that such communities occupy, 

 and their degree of relative maturity, they 

 all agree in having a stratified structure. 



Grassland communities have three verti- 

 cal strata: namely, subterranean, floor, and 

 herbaceous (Vestal, 1913; Cameron, 

 1917). The subterranean stratum has been 

 investigated for substratification of the root 

 systems of grassland plants (Weaver, 1920; 

 Pavlychenko, 1937); such studies show a 

 well-developed system of layering. These 

 subterranean extensions of the herbaceous 

 cover, chiefly grass roots, directly affect 

 other subterranean plants (fungi and bac- 

 teria) and herbivorous animals in making 

 food available. Indirectly they affect aera- 



tion and drainage, and provide a more prof- 

 itable hunting area for carnivores. 



Snails are fewer in number in the grass- 

 lands as compared with forests, and earth- 

 worms are not so common (Pearse, 1939). 

 Insects are among the most abundant ma- 

 croscopic animals of grasslands (Vestal, 

 1913; Wolcott, 1937). Grasshoppers and 

 their allies are notably abundant and 

 characteristic (Uvarov, 1928; Isely, 1938, 

 1938a, 1941), and since many oviposit in 

 the upper portion of the subterranean 

 stratum, the soil is directly utilized for a 

 portion of their life cycles. Burrowing 

 spiders and grassland ants excavate in this 

 stratum. Here also are the burrows of nu- 

 merous grassland mammals (Haviland, 

 1926; Petry and Visher, 1926; Clements 

 and Shelford, 1939, Chap. 8; Hamilton, 

 1939), including a rich fauna of rodents 

 (Table 35). Some rodents (pocket gophers 

 and Asiatic mole-rats) not only have their 

 burrows in the subterranean stratum, but 

 feed in it as well, sometimes tunneling sev- 

 eral feet below the surface of the soil, and 

 feeding on bulbs and roots. Such animals 

 are characteristically adjusted to their spe- 

 cialized habitat with vestigial eyes, stout 

 barrel-shaped bodies, short and powerful 

 limbs with fossorial forepaws, and vestigial 

 tails. 



Other grassland constituents, typical of 

 higher strata, utilize the subterranean por- 

 tion of the community. Bank swallows in 

 the pampas dig their holes in the walls of 

 the extensive entrances of the viscacha 

 burrows and are dependent upon the latter 

 for nesting sites. Many birds (sand 

 martin, rollers, bee-eaters of the Asiatic 

 grassland; cf. Haviland, 1926) and 

 mammals (jack rabbit) breed in holes often 

 deep enough to be subterranean and fur- 

 ther bridge the gap between the floor and 

 subterranean habitats. Subterranean bur- 

 rows of fossorial rodents are used by other 

 animals. For example, the viscacha's bur- 

 row and ancillary trenches are used as 

 nesting sites by the pampas sand mar- 

 tin (Atticora) , burrowing owl, six species 

 of sand wasps, a species of reduviid bug, 

 and a species of nocturnal cicindelid beetle, 

 none of which is commonly found else- 

 where. In North America the prairie rattler 

 (Crofahi?} viridis) is typical of the floor 

 stratum, but since the snake is essentiallv 

 nocturnal and one of its chief foods is 



